The present invention relates generally to a semiconductor device, and in particular, to a semiconductor laser diode having a graded interlayer.
Semiconductor laser diodes have lately been put to practical use in various fields, such as optical communication, multiplex communication, and space communication for the reason that light emitted from the semiconductor laser diodes has a narrow frequency width and a sharp directivity. In addition, the semiconductor laser diodes are widely used in a high-speed laser printer and an optical storage device such as a compact disk player (CDP) and a CDP/recorder.
In particular, a nitride semiconductor laser diode has been noticed as a light source of the optical storage device because the nitride semiconductor laser diode has a direct transition type that ensures a high probability of laser oscillation. For another reason, the nitride semiconductor laser diode provides a short oscillation wavelength which covers from an ultra violet region to a green region due to a wide band gap energy.
In addition, the nitride semiconductor laser diode is increasingly popular in terms of environmental friendliness since arsenic (As) is not used as a main component.
The semiconductor laser diode used as the light source of the optical storage device has to satisfy a single mode and a high-output characteristic. For this, the semiconductor laser diode has a ridge waveguide to limit an injected current, so that a threshold current is reduced and a gain is achieved only when in the single mode.
The nitride semiconductor laser diode emits light when holes injected vertically through a ridge are combined with electrons in an active layer. At this time, when the electrons, which are relatively light in weight, pass intactly through the active layer, a problem arises in that the electrons cannot participate in radiative recombination in the active layer.
In order to address such a problem, an electron blocking layer (EBL) has been introduced. That is, by forming the EBL on an upper portion of the active layer, the number of electrons which cannot participate in the radiative recombination is reduced when the electors intactly pass through the active layer.
FIG. 1 is an energy band diagram for explaining a role of an EBL of a conventional semiconductor laser diode.
Referring to FIG. 1, in order to prevent electrons provided from an n-clad layer from passing a quantum well and then jumping to a p-clad layer, a p-EBL is formed between an active layer and the p-clad layer.
The p-EBL acts as a sort of an energy barrier and thus prevents overflow of electrons. By confining the electrons provided from the n-clad layer within the quantum well active layer, the p-EBL enables the electrons to participate in the radiative recombination.
To function as an effective energy barrier, the p-EBL is composed of AlGaN with a high aluminum (Al) composition (e.g., above 20%).
The quantum well active layer is composed of InGaN. In comparison with the p-EBL composed of AlGaN, the quantum well active layer shows a significant difference in rigidities and lattice parameters. As a result, an energy band has an abrupt gradation, and a strain is generated in an interface between the quantum well active layer and the p-EBL.
Meanwhile, light generated by hole-electron recombination in the active layer of the semiconductor laser diode is amplified while reciprocating between a front cleavage facet and a back cleavage facet. The light is emitted when an oscillation requirement is satisfied.
Since the semiconductor laser diode operates according to the aforementioned principle, important features (e.g., threshold current, reliability, optical efficiency, etc.) of the semiconductor laser diode are dependent on the quality (e.g., roughness, angle, crack, etc.) of the cleavage facet.
The cleavage facet of the semiconductor laser diode is formed through a scribing and breaking process in which a physical strength is applied. Therefore, the quality of the cleavage facet is sensitive to a material's rigidity, a strain, etc.
However, in the conventional semiconductor laser diode, the active layer and the p-EBL have significantly different rigidities and lattice parameters from each other. Therefore, while forming the cleavage facet, as shown in FIG. 1, a crack-shaped defect is formed in the cleavage facet along the interface between the active layer and the p-EBL.